1. Field of the Invention
The present invention relates to a Raman amplifier.
2. Description of the Background Art
In Raman amplifiers, techniques have been proposed that enable the achievement of a flat gain spectrum by properly predetermining each value of the powers of pump lightwaves having a plurality of wavelengths. (See a literature written by Y. Emori et al., “Broadband lossless DCF using Raman amplification pumped by multichannel WDM laser diodes,” Electron. Lett., Vol. 34, No. 22, pp. 2145–46, 1998, for example.) However, in the technique described in the above-described literature, in which each value of the powers of pump lightwaves having a plurality of wavelengths is a constant value, when the power of an input signal varies, a gain variation is created due to a saturation tendency of the gain in Raman amplification.